(1) Field of the Invention
The present invention relates to preparation of an ethylene polymer having a broad molecular weight distribution. More particularly, it relates to an ethylene polymer which has excellent melt properties and processability and is suitable for blow molding and extrusion molding, and to a process for the preparation thereof.
(2) Description of the Related Art
Polyethylene having a relatively high molecular weight and a broad molecular weight distribution is required in the fields of blow molding and extrusion molding. Attempts have been made to prepare a polymer having a broad molecular weight distribution in a single polymerization step while selecting an appropriate catalyst. However, in the case of a catalyst capable of giving polyethylene having a broad molecular weight distribution, it is generally difficult to obtain a sufficiently high productivity, and since the molecular weight distribution is regulated by the properties of the catalyst, polyethylene having a molecular weight distribution suitable for the intended use is not advantageously obtained. Accordingly, a process in which polyethylene having a high molecular weight and polyethylene having a low molecular weight are prepared in succession through at least two stages of polymerization has been considered as the process for overcoming this disadvantage and has been proposed as the multi-stage polymerization process. According to this process, polyethylene having a broad molecular weight distribution and hence, showing a good flowability even through the molecular weight is relatively high can be obtained at a high productivity. To adjust the rigidity of polyethylene, an .alpha.-olefin is copolymerized as the comonomer, and it is known that if the .alpha.-olefin is incorporated so that the .alpha.-olefin content in a polymer having a high molecular weight is higher than in a polymer having a low molecular weight, the balance between the rigidity and the environmental stress cracking resistance (ESCR) is highly improved
However, ethylene polymers prepared by the multi-stage polymerization process have several problems in processing and molding though it has excellent properties as described above. Namely, the melt tension of the polymer is low and the die swell is small. In the case of blow molding, not only the molecular weight and molecular weight distribution but also the melt properties of the polymer, that is, the melt tension and die swell, are important. In the blow molding process, a cylindrical molten polymer called parison is extruded from a die, and when the parison comes to have a predetermined length, a gas such as air is blown into the parison to cause the parison to adhere closely to the inner wall of the mold and obtain a molded article If the melt tension of the polymer is low, drawdown, i.e., hanging-down of the parison due to its own weight occurs, and even if molding of a large-size product is tried, breaking of the parison hanging from the die due to the weight of the parison occurs. When the molten polymer is extruded from the die of the molding machine, swelling occurs due to the Barus effect. The ratio of the diameter of the parison to the diameter of the die is called the die swell ratio and is used as the criterion for indicating the swelling degree. According to the blow molding process, a bottle or another hollow article is formed from this parison having a predetermined length, but in the case of polyethylene having a small die swell, the thickness of the product is reduced and it is difficult to obtain products having a constant weight. Therefore, exchange of dies becomes necessary for adjusting the thickness. Accordingly, spare dies are necessary for makers providing various molded products, and the productivity is reduced and the process becomes disadvantageous from the economical viewpoint. Moreover, the die swell depends on the shear rate, and thus, if the shear rate is changed, also the die swell is changed. If the dependency of the die swell on the shear rate is large, the thickness is changed by a slight change of the shear rate and it becomes difficult to obtain products having a constant weight, and in short, the molding stability is degraded and the process becomes extremely disadvantageous from the industrial viewpoint. Furthermore, where a film is prepared by inflation as the extrusion molding, if the melt tension is low, the bubble becomes unstable.
As the means for improving the die swell, one of visco-elastic properties, there has been proposed a multi-stage polymerization process including three stages of polymerization, in which a polymer having an extremely high molecular weight is prepared at one of the polymerization stages. For example, Japanese Examined Patent Publication No. 59-10724 discloses a process in which a polymer having a molecular weight higher than 1,000,000 is formed in an amount of 1 to 10% based on the entire polymer, and in examples of this patent publication, an attempt to improve the die swell by preparing a polymer having a molecular weight of 3,000,000 in an amount of 5% based on the formed polymer is shown.
Japanese Unexamined Patent Publication No. 57-141409 discloses a process in which a high-molecular-weight polymer having an intrinsic viscosity [.eta.] of 7 to 40 is prepared, and in examples of this patent publication, there is shown an attempt to adjust the die swell in a broad range by changing the intrinsic viscosity [.eta.] within the range of from 7.69 to 14.8 and controlling the amount formed of the polymer below 10.5% based on the entire polymer. However, when we made tracing experiments of these attempts, it was found that the improvement of the die swell by these attempts is still insufficient and the dependency of the die swell on the shear rate tends to increase. Moreover, polyethylene containing a small amount of a polymer having an extremely high molecular weight is insufficient in the homogeneousness because of the presence of hard spots or gels. Moreover, the foregoing proposals do not refer to the melt tension which is one of important characteristics of the melt.
Japanese Unexamined Patent Publication No. 59-227913 discloses a process in which an ultra-high-molecular-weight polymer having an intrinsic viscosity [.eta.] of 11 to 26 as measured at 135.degree. C. in decalin as the solvent is formed in an amount of 5 to 23% by weight at the first stage of polymerization, and subsequently, a polymer having [.eta.] of 0.25 to 1.6 and a polymer having [.eta.] of 2.9 to 5.1 are formed in an optional order, and examples of the batchwise process are illustrated and attempts to improve the mechanical strength are shown. However, a high productivity is not attained by the batchwise process, and in the case of the continuous process in which attainment of a high productivity is possible, it is necessary to solve the problem of heterogeneousness caused by the difference of the residence time among particles, which is not observed in the batchwise process, and many other difficult problems.
Furthermore, Japanese Unexamined Patent Publication Nos. 62-25105 through 62-25109 disclose a continuous multi-stage polymerization process, but in all of examples in these patent publications, the amount formed of an ultra-high-molecular-weight component having an intrinsic viscosity [.eta.] of at least 15 as measured at 135.degree. C. in decalin is smaller than 5% by weight and it is taught that if the amount of this component exceeds 5% by weight, a gel or hard spot is formed.
In the foregoing conventional techniques, since the ratio of the formed ultra-high-molecular-weight component is low or the polymerization is carried out batchwise, the problem of heterogeneousness of the polymer (formation of gel or hard spots) does not arise.
Moreover, Japanese Unexamined Patent Publication No. 58-138719 discloses a process in which a high-molecular-weight ethylene (co)polymer having an intrinsic viscosity [.eta.] of 1.5 to 11, preferably 1.5 to 7, an ethylene (co)polymer having an intrinsic viscosity [.eta.] of 0.5 to 8 and an ethylene (co)polymer having an intrinsic viscosity [.eta.] of 0.2 to 3 are formed by the multi-stage polymerization. In this patent publication, a high-molecular-weight component having a relatively low molecular weight corresponding to [.eta.] of 2.60 to 6.72 is mainly examined in both of the batchwise process and the continuous process, and it is taught that since the product is an ethylene (co)polymer comprising the high-molecular-weight component, the low-molecular-weight component and the medium-molecular-weight component, the process is advantageous in that formation of hard spots is controlled. However, the melt properties necessary for formation of hollow articles are not described. As the result of investigations made by us, it was found that, in the conventional continuous multi-stage polymerization process, if [.eta. ] of the high-molecular-weight component exceeds 6, hard spots are readily formed in the product, and any means must be taken for overcoming this disadvantage.
We already proposed the process in Japanese Unexamined Patent Publication No. 61-207404, but the process is not completely satisfactory in the continuous polymerization. Moreover, since the molecular weights and formation ratios of the respective components are not determined based on the intrinsic viscosity of finally obtained polyethylene, the melt tension, die swell and drawdown resistance are not satisfactorilY improved in the obtained polyethylene and the effect of a radical generator, stated in this patent publication, is not manifested.
As is apparent from the foregoing description, development of a process comprising the step of forming ultra-high-molecular-weight polyethylene by polymerization, in which polyethylene having excellent melt tension, die swell and drawdown resistance is continuously prepared at a high productivity, the obtained polymer has an excellent homogeneousness even if the ratio of the ultra-high-molecular-weight component is high, formation of gels or hard spots is controlled and the obtained polyethylene is suitable for blow molding, is eagerly desired.